Threshold Tear Strength of Elastomers.

Tear strengths have been measured for a wide variety of molecular networks under threshold conditions i.e., at high temperatures, low rates of tearing, and with swollen samples. For all of the polymers examined, the threshold tear strength was found to be proportional to the square root of the average molecular weight M sub c of network strands, in agreement with theory. However, for the same M sub c and hence for similar values of elastic modulus, different polymers showed major differences in threshold tear strength. The tear strength of polydimethylsiloxane networks was only about one-third as large as that for networks of polybutadiene and cis-polyisoprene and the values obtained for polyphosphazene networks were only about one-fifth as large as the same M sub C. These striking differences are attributed to differences in network strand length and extensibility for the same molecular weight. The threshold tear strengths are shown to be in satisfactory quantitative agreement with theoretically predicted values on this basis. Author